Gas mixture separation by distillation with feed-column heat exchange and intermediate plural stage work expansion of the feed



June 23, 1970 Ms'L. HOFFMAN 3,516,261

GAS MIXTURE SEPARATION BY DISTILLATION WITH FEED-COLUMN HEAT EXCHANGE AND INTERMEDIATE PLURAL STAGE WORK EXPANSION OF THE FEED Filed April 21, 1969 2 Sheets-Sheet 1 Z l ql Gem )IVVVVV 6 I I i 4 44 42 4 E: D i 49-2 -4@ do W -32 34' 38 Q4 I "vvvvvw 1 2e :29 act I2 L. 8 $9 2 lilo le)%@ '8 ETHANa 8c HEAA/IEQ METHAHE HVDEOCAQBQNS INVENTOR.

Mum/p75,. L- Ham-Mad June 23, 1970 M. L. HOFFMAN 3,516,261

GAS MIXTURE SEPARATION BY DISTILLATION WITH FEED-COLUMN HEAT EXCHANGE AND INTERMEDIATE PLURAL STAGE wonx EXPANSION OF THE FEED Filed April 21, 1969 I 2 Sheets-Sheet 2 ';QLr*- I04 3: T 34 2 I08 I00 I l I061 I06 I02 I06 9 M/CHflE'L L Hoa /wad INVENTOR.

United States Patent Oflice Patented June 23, 1970 GAS MIXTURE SEPARATION BY DISTILLATION WITH FEED-COLUMN HEAT EXCHANGE AND INTERMEDIATE PLURAL STAGE WORK EX- PANSION OF THE FEED Michael L. Hoffman, Los Angeles, Calif., assignor to McDonnell Douglas Corporation, Santa Monica, Calif., a corporation of Maryland Filed Apr. 21, 1969, Ser. No. 817,903 Int. Cl. Fj 3/02 U.S. C]. 62-24 16 Claims ABSTRACT OF THE DISCLOSURE Method and system for separating components of gas mixtures such as mixtures of hydrocarbons containing from 1 to about 4 carbon atoms, and particularly designed for recovery of at least 80% of the ethane present in a high pressure stream of natural gas containing chiefly methane and minor amounts of ethane and higher hydrocarbons such as propane, without requiring an external refrigeration system, which involves, according to one embodiment, passing a major first portion of the high pressure feed mixture in heat exchange relation with the contents of the lower portion of a distillation column at a plurality of different temperature levels to cool the first portion of the feed mixure, cooling a second portion of the feed mixture by means of cold overhead methane withdrawn from the column, combining the cooled first and second portions of the feed mixture and subjecting the resulting combined feed mixture to a first work expansion to an intermediate pressure and to further reduce the temperature of the feed mixture. A major portion of the thus Work expanded feed mixture is passed in heat exchange relation with the contents of the upper portion of the distillation column at a plurality of different temperature levels to further cool the last mentioned portion of feed mixture, and the remaining portion of work expanded feed mixture is additionally cooled by cold methane gas withdrawn from the top of the column, and is combined with the additionally cooled first major portion of feed mixture withdrawn from heat exchange relation with the upper portion of the column. At least a portion of the resulting additionally cooled partially liquefied combined feed mixture is subjected to a second work expansion to a lower pressure and further reduction of temperature, and is fed to the top of the distillation column. A non-adiabatic distillation of the gas mixture in the column occurs, and a liquid bottoms product is withdrawn consisting essentially of ethane and higher hydrocarbon, and an overhead consisting essentially of methane is withdrawn from the top of the column and is used for cooling portions of the high pressure feed mixture and of the initially expanded feed mixture, as noted above.

This invention relates to the separation of components of gas mixtures having different boiling points, such as the components of hydrocarbon mixtures containing hydrocarbons of from 1 to about 4 carbon atoms, particularly natural gas mixtures containing methane, ethane and in some cases small amounts of propane and heavier hydrocarbons, and is especially concerned with a procedure and a system for the high recovery of ethane from a high pressure stream of natural gas, containing a major proportion of methane and a minor proportion of ethane, by low temperature fractional distillation employing nonadiabatic distillation in the column and employing column refrigeration to permit useful cooling of the feed mixture by heat exchange with the column contents at several temperature levels, and utilizing the energy of the high pressure natural gas feed to permit work expansion of the feed in conjunction with the refrigeration by the contents of the column, to obtain sufiicient refrigeration of the feed without the employment of an external refrigeration system.

In most natural gas mixtures there are appreciable amounts of hydrocarbons of higher molecular weight than methane, the major constituents usually present in natural gas. It is usually desirable to remove these hydrocarbons as liquids and to market them separately since in general they have greater value as separate components than as constituents of natural gas. Thus, for example, hydrocarbons heavier than methane, particularly ethane, contained in natural gas are usually worth substantially more than their heating value.

In the case of the heaviest components such as pentane and higher molecular weight hydrocarbons, removal thereof may be accomplished by fractional cooling and collecting of the respective condensates. In the case of hydrocarbons having molecular weight between methane and pentane, that is, methane, ethane, propane and butane, a more complicated processing procedure is generally required. Among the processes generally used for separation of the latter components from natural gas is low temperature fractional distillation of the natural gas.

In convenional processes for such low temperature frac tional distillation, the usual adiabatic distillation generally is employed, involving the mixing of hot and cold liquids or vapors in the column with high temperature differences between the various components. Due to the fact that there is a very large temperature difference between the boiling point of methane and other hydrocarbons such as ethane and propane, adiabatic distillation results in large thermodynamic irreversibilities within the distillaton column and consequently poor thermodynamic efliciency.

Thus, the processing of natural gas for recovery of ethane from natural gas mixtures containing a major proportion of methane and a minor proportion of ethane, has heretofore been too expensive to warrant ethane recovery greater than about of the ethane present in the original feed mixture. This is primarily due to the fact that in the above noted conventional procedure of thermodynamic inefficiency, a large amount of external refrigeration is required in order to liquefy enough of the feed stream to permit fractionation.

According to the present improved process and system, at least of the ethane from a high pressure natural gas stream consisting chiefly of methane and containing at least 4% ethane, can be recovered, without requiring an external refrigeration system. In the event that hydrocarbons heavier than ethane are present, e.g. propane, these are recovered together with ethane in the liquid fraction.

The present invention is a substantial improvement over the above noted prior art processes in that the invention process involves the recovery and removal of refrigeration from the column along the length thereof, at various temperature levels, and utilizing such refrigeration for cooling of a substantial portion of the natural gas feed mixture. The passage of a substantial portion of the feed mixture in heat exchange relation with the column contents at a plurality of temperature levels in the lower and upper portions of the column, abstracts heat and provides reboil for the column along its length, and effects a nonadiabatic distillation in the column, resulting in an efficient utilization of refrigeration from the column. In conjunction with such refrigeration from the column, the energy in the high pressure, e.g. 1,000 p.s.i., natural gas feed stream is utilized by carrying out a first work expansion of the feed stream at an intermediate stage between its passage in heat exchange relation with the contents of the column in the lower portion thereof and the contents of the column in the upper portion thereof, and carrying out a second work expansion of at least a portion of the feed stream following its passage in heat exchange relation with the column contents, to a reduced pressure corresponding to the pressure maintained in the column, to further reduce the temperature of the feed steam, the resulting further expanded and liquefied feed stream then being introduced as feed to the top of the column. Further cooling efficiency is effected by passing the cold, e.g. methane, overhead from the column in heat exchange relation with portions of the feed stream.

Thus there is provided according to the invention, a process and system for separating the components of a gas mixture, particularly a mixture of hydrocarbons containing from 1 to about 4 carbon atoms, having different boiling points, by low temperature rectification, in the absence of external refrigeration, to produce a bottoms product consisting essentially of the highest boiling point component of said mixture and an overhead of the remaining components of said mixture, which comprises dividing said high pressure feed mixture into a first portion and a remaining portion, passing said first portion of said high pressure feed mixture along the lower portion of a distillation column in heat exchange relation with the contents of said lower portion of said column at a plurality of temperature levels, said first portion of the feed mixture being cooled by the contents of said column and providing reboil heat to the lower portion of said column, withdrawing said cooled first portion of the feed mixture from heat exchange relation with the contents of the lower portion of said column, cooling said remaining portion of said high pressure feed mixture externally of said column, combining said cooled first and said cooled remaining portions of said high pressure feed mixture, work expanding said combined feed mixture to an intermediate pressure and further cooling said feed mixture, passing a first portion of said further cooled expanded feed mixture along the upper portion of said distillation column at a plurality of temperature levels, said last mentioned first portion of feed mixture being additionally cooled by the contents of said column and providing reboil heat to the upper portion of said column, withdrawing said last mentioned additionally cooled portion of feed mixture from heat exchange relation with the contents of the upper portion of said column, additionally cooling the remaining portion of said further cooled expanded feed mixture externally of said column, combining said additionally cooled remaining expanded portion of said feed mixture with said additionally cooled expanded feed mixture withdrawn from heat exchange relation with the contents of the upper portion of said column, further work expanding at least a portion of said last mentioned combined feed mixture and further reducing the temperature thereof, introducing the resulting further expanded and cooled liquefied feed mixture into the top of said column as feed, eflecting a separation of said feed mixture in said column by a non-adiabatic distillation in said column, withdrawing a liquid bottoms product consisting essentially of the highest boiling point components of said feed mixture, withdrawing a cold overhead product of the remaining components in said feed mixture, and passing said cold overhead product in heat exchange relation with said above noted remaining portions of said feed mixture for said cooling and additional cooling thereof.

For effecting non-adiabatic distillation in the column and cooling of the feed mixture as described above, the feed mixture can be passed through one or more heat exchangers positioned within and along the lower portion of the column, and through one or more heat exchangers positioned within and along the upper portion of the column, for heat exchange with the vapor-liquid mixture in the column at a plurality of temperature levels in the lower and upper portions thereof, or alternatively such feed mixture can be passed through a plurality of separate heat exchangers positioned externally of and along the respective lower and upper portions of the column, in heat exchange relation with vapor-liquid mixture removed from various temperature levels in the respective lower and upper portions of the column and passed through such separate heat exchangers and the partially vaporized exiting mixtures injected back into the column at a plurality of appropriate temperature levels therein.

Where the column employs the usual plates or trays, the vapor-liquid mixture can be removed from a plurality of such trays, and then injected back into the column at substantially the same or a slightly lower level from which such vapor-liquid mixtures were removed, after passage thereof through the external heat exchangers. In either case, that is, by use of either internal or external heat exchangers, substantial refrigeration of the feed mixture is provided by the contents of the column, with reboil being supplied to the column by the feed mixture along the length of the column.

The present invention is particularly applicable for processing natural gas streams containing methane and sufficient quantities of such heavier hydrocarbons as ethane, propane and butane to warrant their separation and recovery as a separate product. Thus, the invention is particularly designed to separate ethane from natural gas mixtures containing a major portion of methane, eg about to about 95% methane, and a minor proportion of ethane, e.g. about 4 to about 10% ethane, and such natural gas mixtures may contain propane together with heavier hydrocarbons such as butane, in an amount of 0 to about 10%, and small amounts of light gases such as hydrogen and nitrogen can also be present to the extent of 0 to about 25% of the total feed, without materially affecting the invention process. These latter light gases will remain with the methane fraction obtained in the process.

Thus, for example, according to the invention process and system, a high pressure stream of natural gas at a pressure of between about 800 and about 1200 p.s.i.a., and having the composition noted above can be separated into two fractions, a liquid fraction containing at least of the ethane present in the feed mixture and a fraction containing about 98 to about 99.5% methane. Where such initial feed mixture contains about 6% ethane an recovery of the ethane present in the original feed mixture can be achieved, and if the concentration of the ethane in the feed is somewhat lower, e.g. of the order of about 4 to 5%, an 85 to recovery of the ethane present in the feed can be realized.

By the term high pressure feed stream as employed in the specification and claims, is meant a feed stream having a pressure in excess of about 800 p.s.i.a.

An illustrative process for producing the improved results of the invention is described below, for processing a stream of natural gas at 1,000 p.s.i.a., and consisting of 90% methane, 6% ethane and 4% propane and higher hydrocarbons, to remove approximately 85 of the ethane present'in the feed and substantially all of the heavier hydrocarbons as a separate liquid fraction, in connection with the accompanying drawing wherein:

FIG. 1 is a schematic representation of a preferred form of a separation system according to the invention; and

FIG. 2 illustrates an alternative means for exchanging heat between the column contents and the feed stream.

Referring to FIG. 1 of the drawing, the above noted exemplary natural gas feed mixture or stream, provided at 10, at a pressure of 1,000 p.s.i.a. and a temperature of 580 R. (Rankin) and free of carbon dioxide and water, is divided into two streams 12 and 14. The larger stream 12, amounting to approximately 86% of the feed, is cooled first by passage through heat exchanger coil 16 in the reboiler 18 of a fractionating or stripping column having approximately 23 stages or plates, and is then further cooled by passage through a heat exchanger coil 22 about the fourth stage in the lower portion 24 of the column, to a temperature of about 550 R. The smaller stream 14, about 14% of the feed, is cooled by passage through coil 26 of a heat exchanger 28 by outgoing methane passing through coil 30 of such heat exchanger.

The two resulting cooled streams of feed gas at 29 and 31, are recombined at 32 and are again split into two streams 34 and 36. The smaller stream 34 amounting to about 22% of the feed, is successively cooled by passage through heat exchanger coils 38, and 42, located within the lower portion of the column at the seventh, tenth and thirteenth stages, respectively, to a temperature of about 445 R. for the exiting stream at 44. The larger feed stream 36, about 78% of the feed, is cooled by passage through coil 46 of heat exchanger 48 by cold overhead outgoing methane gas passing through coil 49. The resulting two further cooled streams 44 and 51 are again combined at 52 and are work expanded by the turbine or expander 54 to a discharge pressure at 56, of 580 p.s.i.a. and a reduced temperature of 409 R.

The expanded feed discharge at 56 is again divided into two streams 58 and 60. The larger stream 58 containing 60% of the feed, is additionally cooled by passage through heat exchanger coils 62, 64 and 66, located above the 18th, 20th and 22nd stages in the upper portion 67 of the column 20. The smaller stream 60, amounting to about 40% of the feed, is additionally cooled by passage through coil 68 of heat exchanger 70 in heat exchange relation with cold overhead methane from the column 20, passing through coil 72.

The resulting additionally cooled feed gas streams 74 and 76, both at temperature of 357 R., are combined at 78, and as result of the cooling of the feed streams as described above, the combined feed stream at 78 is now approximately 37% liquid. The partially liquid feed stream 78 is then separated in a separator 80 into a liquid fraction 82 containing about 12.6% ethane and substantially all the heavier hydrocarbons in the feed stream, and a vapor fraction 84 containing 1.7% ethane with the remainder methane, and any of the lighter gases present in the feed.

The vapor stream 84 is fed to a second turbine or expander 86 where it is work expanded to produce an expanded vapor discharge at 87 at a reduced pressure of 350 p.s.i.a., the pressure within column 20, and a reduced temperature of about 320 R. The resulting expanded vapor at 87 is then fed to another separator 88 where liquid formed during the work expansion is removed at 89, amounting to about 4% of the feed. The liquid at 82 is sub-cooled by passage through coil 90 of a heat exchanger 91 in heat exchange relation with overhead cold methane vapor from the column, passing through coil 92, and the discharge from coil 90, at about 325 R., is then throttled at 93 to a pressure of 350 p.s.i.a., and the throttled liquid feed stream at 94 is combined with the liquid feed stream 89, and the combined liquid 95, containing substantially all of the ethane in the initial feed stream, is fed into the top of column 20 and used as feed therein.

As previously noted, the compressed cooled feed passing through heat exchanger coils 16, 22, 38, 40 and 42 in the lower portion 24 of the column, and the further cooled and expanded feed passing through the heat exchange coils 62, 64 and 66 in the upper portion 67 of the column, provide for addition of heat to both the lower and upper portions of the column at various temperature levels and concurrent refrigeration of the feed at different temperature levels both in the lower and upper portions of the column, allowing column refrigeration to be available to the feed at temperature sufficiently low to permit elficient cooling of the feed passing through the heat exchanger coils, and effecting a. non-adiabatic distillation in the column. It will be noted that the entire column 20 functions as a stripping column with the substantially liquid feed mixture at 95 introduced into the top of the column being sufiiciently cooled to prevent any substantial amount of ethane being carried out as vapor into the overhead methane at 97.

In column 20, the feed stream 95 introduced into the column, is stripped of ethane and heavier hydrocarbon by rising vapors generated by the addition of heat from the above described heat exchanger coils located in the column. The ethane and heavier hydrocarbons, about 10% of the feed, are removed as liquid at 96 from. the bottom of the column, the liquid recovered at 96 containing about 85% of the ethane present in the initial feed stream at 10.

Overhead vapor removed from the column at 97, and containing substantially pure methane and less than about 1% ethane, and at a temperature of about 325 R., is combined with the feed vapor stream 98 from separator 88, also containing substantially pure methane, and the combined cold natural gas stream 99*, is employed to provide cooling in heat exchangers 91, 70, 48 and 30, as previously noted, and is discharged at 99'.

An important advantage of the invention, as illustrated by the above described embodiment, is that by locating the column heat exchangers 16, 22, 38, 40 and 42; and 62, 64 and 66 at several different temperature levels in the lower and upper portions of the column, refrigeration is removed from the column at several levels both at the bottom and top of the column, which thus enables utilization of substantially all of the refrigeration available from the column in cooling the feed, with only a small portion of refrigeration available from the column being removed in reboiler 18 of the column. On the other hand, in conventional procedure where all of the heat required by the column is provided in reboiler at the bottom of the column, since the temperature in the reboiler, e.g. is about 560 R. in the above embodiment, essentially no refrigeration is available to cool the feed, and the cooling thus provided by the heat exchangers at various temperature levels within the column according to the invention, would require replacement by an external refrigeration system.

Further, as an additional important cooperating feature of the invention, it is noted in the specific embodiment described above that additional refrigeration is provided by utilizing the energy of the high pressure feed at 10 to permit work expansion of the feed stream at 54 and at 86, to thereby provide essentially all of the additional refrigeration required in conjunction with the above noted column refrigeration, for suflicient cooling of the feed mixture to liquefy same and permit introduction thereof as feed into the top of the column. It will be seen that all of the feed mixture is expanded through at least one of the work expanders 54 and 86, namely expander 54 in the embodiment of FIG. 1.

Further cooling efficiency is provided, as described above in relation to the embodiment illustrated in FIG. 1, by utilizing the cold overhead substantially pure methane, combined with a minor portion of the expanded chiefly methane vapor at 98, to provide further cooling for portions of the feed mixture.

Instead of passing the feed in a continuous manner through heat exchangers in the lower and upper portions of the column in indirect heat exchange relation with the column contents, as illustrated in FIG. 2, heat may be exchanged between the column and the feed streams 12 and 58 by removing liquid from the various trays or stages as described above and illustrated in FIG. 1 at a multiplicity of temperature levels both in the lower and upper portions of the column 20, as indicated at 100, and partially, evaporating such liquid by passage through coils 102 of a series of external heat exchangers 104 and 104 positioned in vertically spaced relation along the lower and upper portions, respectively, of the column, in heat exchange relation with the feed passing through coils 106 of such heat exchangers, the vapor-liquid mixture exiting coils 102 then being injected back into the column at substantially the same tray level from which the respective liquids were withdrawn, as indicated at 108. Thus, in FIG. 2, the feed stream is also cooled at a multiplicity of temperature levels or stages along the length of the column, by refrigeration supplied from the column non-adiabatically, as in the case of the column embodiment shown in FIG. 1.

Hence, the expression passing the feed mixture in heat exchange relation along the lower portion and along the upper portion of the distillation column, as employed herein, is meant to cover the embodiments of both FIGS. 1 and 2, and denote passage of the feed stream in heat exchange relation with the contents of the column at a plurality of temperature levels along the column, regardless as to whether the feed stream is introduced into heat exchangers positioned internally in the column for heat exchange therewith, or is passed externally of the column through several external heat exchangers in heat exchange relation with the liquid contents removed from various levels within the column and returned thereto, as in FIG. 2.

It will, of course, be understood that the specific number of heat exchanger coils located within the column, as illustrated in the embodiment of FIG. 1, or the number of heat exchangers located externally of the column with the corresponding number of column liquid removal and injection points in the respective lower and upper portions of the column, can be varied, with the effect of increasing or decreasing the approach to complete reversibility, the greater the number of such heat exchangers and liquid removal and injection points, the closer the approach to reversibility. Further, if desired, combinations of internal and external column heat exchangers of the type illustrated in FIGS. 1 and 2, can be employed, e.g. internal column heat exchanger coils as illustrated at 16, 22, 38, 40 and 42 in FIG. 1 can be employed in the lower portion of the column, and a plurality of external heat exchangers as indicated at 104 in FIG. 2, in the upper portion of the column; or internal heat exchanger coils such as 62, 64 and 66 can be employed in the upper portion of the column and a plurality of external heat exchangers as indicated at 104 in FIG. 2, in the lower portion of the column. In preferred practice, refrigeration from the fractionating column is recovered at at least four different temperature levels.

From the foregoing, it is seen that the invention provides a process and system for the separation of the components of a gas mixture, and is designed particularly for the separation of hydrocarbons, especially for recovery of the major portion of the ethane in a methane-ethane high pressure natural gas stream consisting essentially of these two components, with high efficiency, by recovery of a major portion of the refrigeration within the distillation column at relatively low temperatures along the length of the column, and effecting a non-adiabatic distillation, in conjunction with the utilization of the energy of the high pressure feed stream to provide essentially the balance of the required refrigeration by work expansion of said high pressure stream, thus avoiding the necessity of external refrigeration.

The process and system of the invention can also be employed for separating other gas mixtures, such as a high pressure mixture of hydrogen, methane, ethylene, ethane and heavier hydrocarbons into a light fraction containing hydrogen and methane with a small amount of ethylene, and a heavy fraction containing ethylene, ethane and heavier hydrocarbons.

While we have described particular embodiments of our invention for the purpose of illustration, it should be understood that various additional modifications and adaptations thereof may be made within the spirit of the invention, and hence the invention is not to be taken as limited except by the scope of the appended claims.

I claim:

1. A process for separating the components of a high pressure stream of a gas mixture having different boiling points, by low temperature rectification, in the absence of external refrigeration, to produce a bottoms product consisting essentially of the highest boiling point component of said mixture and an overhead of the remaining components of said mixture, which comprises passing a substantial portion of said high pressure feed mixture in heat exchange relation with the contents of a distillation column for rectification of said gas mixture, at a plurality of temperature levels in the lower and upper portions of said column, said portion of said feed mixture being cooled by the contents of said column and providing reboil heat to said column and effecting a non-adiabatic distillation in said column, subjecting said high pressure feed mixture to a first work expansion at an intermediate stage between passage of said portion of said feed mixture in heat exchange relation with the contents of the column in the lower portion thereof and the contents of the column in the upper portion thereof, and subjecting at least a portion of the initially expanded feed stream following passage of said substantial portion thereof in heat exchange relation with the column contents, to a second work expansion, introducing the resulting further expanded and liquefied feed stream as feed into the top of said column, withdrawing a liquid bottoms product consisting essentially of the highest boiling point components of said feed mixture, and withdrawing a cold overhead product of the remaining components in said feed mixture.

2. A process as defined in claim 1, including passing said cold overhead product in heat exchange relation with portions of said feed mixture for additional cooling thereof.

3. A process as defined in claim 1, wherein said feed gas mixture is a natural gas containing a major proportion of methane and a minor proportion of ethane.

4. A process for separating the components of a high pressure stream of a gas mixture having different boiling points, by low temperature rectification, in the absence of external refrigeration, to produce a bottoms product consisting essentially of the highest boiling point component of said mixture and an overhead of the remaining components of said mixture, which comprises dividing said high pressure feed mixture into a first portion and a remaining portion, passing said first portion of said high pressure feed mixture along the lower portion of a distillation column in heat exchange relation with the contents of said lower portion of said column at a plurality of temperature levels, said first portion of the feed mixture being cooled by the contents of said column and providing reboil heat to the lower portion of said column, withdrawing said cooled first portion of the feed mixture from heat exchange relation with the contents of the lower portion of said column, cooling said remaining portion of said high pressure feed mixture externally of said column, combining said cooled first and said cooled remaining portions of said high pressure feed mixture, work expanding said combined feed mixture to an intermediate pressure and further cooling said feed mixture, passing a first portion of said further cooled expanded feed mixture along the upper portion of said distillation column at a plurality of temperature levels, said last mentioned first portion of feed mixture being additionally cooled by the contents of said column and providing reboil heat to the upper portion of said column, withdrawing said last mentioned additionally cooled portion of feed mixture from heat exchange relation with the contents of the upper portion of said column, additionally cooling the remaining portion of said further cooled expanded feed mixture externally of said column, combining saidadditionally cooled remaining expanded portion of said feed mixture with said additionally cooled expanded feed mixture withdrawn from heat exchange relation with the contents of the upper portion of said column, further work expanding at least a portion of said last mentioned combined feed mixture and further reducing the temperature thereof, introducing the resulting further expanded and cooled liquefied feed mixture into the top of said column as feed, eifecting a separation of said feed mixture in said column by a non-adiabatic distillation in said column, withdrawing a liquid bottoms product consisting essentially of the highest boiling point components of said feed mixture, withdrawing a cold overhead product of the remaining components in said feed mixture, and passing said cold overhead product in heat exchange relation with said above noted remaining portions of said feed mixture for said cooling and additional cooling thereof.

5. A process as defined in claim 4, wherein said feed gas mixture is a natural gas containing a major proportion of methane and a minor proportion of ethane.

6. A process as defined in claim 5, wherein said feed gas mixture is natural gas at a pressure of between about 800 and about 1200* p.s.i.a., containing about 75 to about 95% methane, about 4 to about 10% ethane, and to about heavier hydrocarbons, said liquid bottoms product containing chiefly ethane, and said overhead product being chiefly methane, said liquid bottoms product containing at least 80% of the ethane present in said feed gas mixture.

7. A process as defined in claim 4, wherein said first portion of said high pressure feed mixture is passed along and through said lower portion of the distillation column in heat exchange relation with the contents of said lower portion of said column at said plurality of temperature levels, and wherein said first portion of further cooled expanded feed mixture is passed along and through the upper portion of said distillation column in heat exchange relation with the contents of said upper portion of said column at said plurality of temperature levels.

8. A process as defined in claim 4, wherein said first portion of said cooled high pressure feed mixture is passed along the lower portion of said distillation column externally of said column, in heat exchange relation with liquid removed from a plurality of temperature levels within the lower portion of said column, said liquid being partially vaporized during heat exchange relation with said feed, and the resulting vapor-liquid mixtures being introduced back into the column at a plurality of levels therein, and wherein said first portion of further cooled expanded feed mixture is passed along the upper portion of said distillation column externally of said column in heat exchange relation with liquid removed from a plurality of temperature levels within said upper portion of said column, said liquid being vaporized during said heat exchange relation with said feed mixture, and returned to said column at a plurality of levels therein.

9. A process as defined in claim 4, including passing said first portion of said high pressure feed mixture in two stages in heat exchange relation with the contents of the lower portion of the distillation column, and after initially cooling said first portion of feed mixture with the contents of the lower portion of said column, withdrawing said feed mixture from said heat exchange relation and recombining same with the initially cooled remaining portion of said feed mixture, again dividing out a first portion of feed mixture and passing same in heat exchange relation with the contents of the lower portion of said column to further cool said first portion, further cooling the remaining portion of the last mentioned divided portion of the feed mixture, and then combining said further cooled first portion and said further cooled remaining portion of said high pressure feed mixture prior to said work expansion of said combined feed mixture to said intermediate pressure.

V 10. A process as defined in claim 4, including separating the partially liquefied additionally cooled expanded combined feed mixture into a first vapor stream and a first liquid stream, carrying out said further work expansion of the separated vapor stream, separating said further work expanded vapor into a second vapor stream and a second liquid stream, subcooling and throttling said first liquid stream, combining said first and second liquid streams, said combined liquid streams being said feed introduced into the top of said column, and combining said second vapor stream and said cold overhead product from said column, and passing said resulting combined cold vapor stream in heat exchange relation with said first liquid stream for said subcooling thereof, and in heat exchange relation with said remaining portions of said feed mixture for said cooling and additional cooling thereof.

11. A process as defined in claim 9, including separating the partially liquefied expanded combined feed mixture into a first vapor stream and a first liquid stream, carrying out said further work expansion of the separated vapor stream, separating said further work expanded vapor into a second vapor stream and a second liquid stream, subcooling and throttling said first liquid stream, combining said first and second liquid streams, said combined liquid streams being said feed introduced into the top of said column, and combining said second vapor stream and said cold overhead product from said column, and passing said resulting combined cold vapor stream in heat exchange relation with said first liquid stream for said subcooling thereof, and in heat exchange relation with said remaining portions of said feed mixture for said cooling thereof.

12. A process as defined in claim 11, wherein said feed gas mixture is natural gas at a pressure of between about 800 and about 1200 p.s.i.a., containing about to about 95% methane, about 4 to about 10% ethane, and 0 to about 10% heavier hydrocarbons, said liquid bottoms product containing chiefly ethane, and said overhead product being chiefly methane, said liquid bottoms prod uct containing at least of the ethane present in said feed gas mixture.

13. A system for separating the components of a gas mixture having different boiling points, by low temperature rectification in the absence of external refrigeration, to

produce a bottoms product consisting essentially of the highest boiling point component of said mixture and an overhead of the remaining components of said mixture, which comprises means for dividing a high pressure feed gas mixture of said components into a first portion and a remaining portion, a distillation column, first heat exchange means for passing said first portion of said high pressure feed mixture in heat exchange relation with the contents of said distillation column for rectification of said gas mixture, at a plurality of temperature levels in the lower and upper portions of said column for cooling said first portion of the feed mixture, means for withdrawing said cooled first portion of the feed mixture from said first heat exchange means, means for subjecting said high pressure feed mixture to a first work expansion at an intermediate stage between passage of said substan tial portion of said feed mixture in heat exchange relation with the contents of said column in the lower portion thereof and the contents of the column in the upper portion thereof, means for subjecting at least a portion of the initially expanded feed stream following its passage in heat exchange relation with the column contents, to a second work expansion, means for introducing the resulting further expanded and liquefied feed stream as feed into the top of said column, means for withdrawing a liquid bottoms product consisting essentially of the highest boiling point components of said feed mixture, and means for withdrawing an overhead product of the remaining components in said feed mixture.

14. A system, as defined in claim 13, including means for passing said cold overhead product in heat exchange relation with portions of said feed mixture for additional cooling thereof.

15. A system for separating the components of a high pressure stream of a gas mixture having different boiling points by low temperature rectification, in the absence of external refrigeration, to produce a bottoms product consisting essentially of the highest boiling point component of said mixture and an overhead of the remaining components of said mixture, which comprises means for dividing said high pressure feed mixture into a first portion and a remaining portion, a distillation column, a first heat exchange means for passing said first portion of the high pressure feed mixture along the lower portion of said distillation column in heat exchange relation with the contents of said lower portion of said distillation column at a plurality of temperature levels, for cooling said first portion of the feed mixture, means for withdrawing said cooled first portion of the feed mixture from said first heat exchange means, a second heat exchange means for cooling the remaining portion of said high pressure feed mixture externally of said column, means for combining said cooled first and said cooled remaining portions of said high pressure feed mixture, a first work expansion means for work expanding said combined feed mixture to an intermediate pressure and further cooling said feed mixture, a third heat exchange means for passing a first portion of said further cooled expanded feed mixture along the upper portion of said distillation column at a plurality of temperature levels, for additionally cooling said last mentioned first portion of feed mixture, means for withdrawing said last mentioned additionally cooled portion of feed mixture from said third heat exchange means, a fourth heat exchange means for additionally cooling the remaining portion of said further cooled expanded feed mixture externally of said column, means for combining said additionally cooled remaining expanded portion of said feed mixture with said additionally cooled expanded first portion of feed mixture withdrawn from said third heat exchange means, a second work expansion means for work expanding at least a portion of said last mentioned partially liquefied combined feed mixture and further reducing the temperature thereof, means for introducing the resulting further expanded and cooled liquefied feed mixture into the top of said column as feed, means for withdrawing a liquid bottoms product consisting essentially of the highest boiling point components of said feed mixture, means for withdrawing a cold overhead product of the remaining components in said feed mixture, and means for passing said cold 12 overhead product through said fourth and said second heat exchange means in heat exchange relation with said above noted remaining portions of said feed mixture for said cooling and additional cooling thereof.

16. A system as defined in claim 15, including means for separating said partially liquefied combined feed mixture prior to said second work expansion, into a first vapor stream and a first liquid stream, means for conducting said first vapor stream to said second work expansion means, means for separating the discharge of said second work expansion means into a second vapor stream and a second liquid stream, a fifth heat exchange means for subcooling said first liquid stream, valve means for throttling said subcooled first liquid stream, means for combining said first and said second liquid streams for introducing same as feed into the top of said column, means for combining said second vapor stream and said overhead product stream, and means for passing said last mentioned combined vapor product stream through said fifth heat exchange means in heat exchange relation with said first liquid stream for subcooling same, prior to passage of said combined vapor product stream through said fourth and said second heat exchange means in heat exchange relation with said remaining portions of said feed mixture for said cooling and additional cooling thereof.

References Cited UNITED STATES PATENTS 1,460,545 7/ 1923 Haynes et al. 6240 2,503,265 4/1950 Haynes 6228 2,661,608 12/1953 Pavlis 6238 2,675,883 4/1954 Deanesly 62-38 2,713,780 7/1955 Williams 6234- 3,280,060 11/1966 Hays 62-24 WILBUR L. BASCOMB, JR., Primary Examiner US. Cl. X.R. 6227, 34, 38 

